import java.util.*;


public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @return int整型
     */
    public static void main(String[] args) {
        Solution solution = new Solution();
        Scanner scanner = new Scanner(System.in);
        int i = solution.jumpFloorII(scanner.nextInt());
        System.out.println(i);
    }
    public int jumpFloorII (int number) {
        // write code here
        int a=0;
        int b=1;
        int num=a+b;
        int sum =1;
        if(number<=1){
          return 1;
        }
        else {
            for (int i = 2; i <=number; i++) {

                a=b;
                b=num;
                num=a+b;
                sum+=b;
            }
            return sum;
        }
    }
}